1. Field of the Invention
The invention relates to dehumidification systems using a compression-based refrigeration cycle.
2. Description of Related Art
The basic components of a compression-based refrigeration cycle are a compressor, a condenser, an expansion valve, an evaporator, and a refrigerant (a volatile liquid). Compression-based refrigeration cycles work because of a combination of physical laws common to all liquids. First, the temperature at which a liquid boils decreases as the ambient pressure decreases. Second, it takes heat to boil (vaporize) a liquid. A liquid vaporizing because of a reduction in ambient pressure absorbs heat from its surroundings; if the vapor is subsequently compressed enough to condense back to a liquid, it gives off heat as it condenses.
A compressor, the active element in the cycle, forces refrigerant to circulate. A compressor pulls cool, low-pressure refrigerant vapor out of the evaporator and compresses it, raising both the pressure and temperature of the refrigerant vapor. This hot compressed refrigerant vapor then flows into a condenser.
A condenser, the high pressure side of the cycle, contains both hot vapor and liquid refrigerant. Because of its high pressure, the vapor refrigerant condenses at a high temperature, expelling heat to the air around the condenser. This resulting warm, pressurized liquid refrigerant then flows to and through an expansion valve.
An expansion valve is a flow restriction which allows the evaporator to be at a lower pressure than the condenser. As warm, high-pressure liquid refrigerant from the condenser flows through the expansion valve towards the evaporator, the pressure drops. Some of the refrigerant boils, cooling the resulting mixture of liquid and vapor refrigerant as it flows to an evaporator.
An evaporator, the low pressure side of the cycle, contains both cold liquid and vapor refrigerant. Because of its low pressure, the liquid refrigerant evaporates (boils) at a low temperature, absorbing heat from the air around the evaporator. The cool vapor is pulled out of the evaporator by the compressor (keeping the evaporator pressure low), thus completing the cycle.
A compression-based refrigerant cycle dehumidifier dries the air because the evaporator is colder than the dew point of the air around it: some of the moisture in the air condenses out onto the evaporator. The resulting liquid water being removed is called “condensate” and typically drips down to be caught in a tray or basin. The air, now cooler and dryer, flows to and cools the condenser. Some of the air leaving the dehumidifier typically passes over and cools the fan motor and the compressor motor.
A dehumidifier's electrical system typically includes a humidity sensor, a set-point adjuster, a compressor motor, and a fan motor. The electrical system often includes one or more of the following: an “off” switch built into the high end of the set point adjuster, a temperature sensor to shut off the dehumidifier (or at least the compressor) if potential icing conditions are detected, and (in units without a drain line) a condensate-level float switch or catch basin weight sensor to shut off the dehumidifier if the catch basin fills up with condensate.
Air about to enter the evaporator is typically drawn through a filter first, to prevent fibers and particles from being trapped by the wet surfaces and clogging the evaporator.
U.S. Pat. No. 2,130,092 discloses a dehumidifier virtually indistinguishable from modern units except for the control electronics and the refrigerant composition. The dehumidifier comprises a refrigerant loop, an air flow path, and an electrical system. The refrigerant loop is a compression-based refrigeration cycle, as previously described. The air passes across the chilled evaporator, cooling the air: the drop in air temperature causes some of the moisture in the air to condense out on the evaporator; this condensate drips down onto a catch pan. The cooled, dehumidified air is then pulled across the hot condenser, dissipating the heat from the compressed refrigerant. The details mentioned include a “finned evaporator” and an “air-cooled fin-type condenser” shown with its refrigerant inlet section adjacent and thermally coupled to its refrigerant outlet section.
U.S. Pat. No. 5,901,565 claims a fan within an orifice plate between the evaporator and the condenser; the fan and orifice plate inhibit radiant heat transfer from the condenser to the evaporator. In addition, it discloses a bypass opening downstream of the evaporator and upstream of the fan and fan motor. The bypass inlet allows some room air to enter the dehumidifier immediately after the evaporator, where it mixes with the cold dehumidified air. The mixture of dehumidified air and bypass air then goes through and cools the condenser. Entraining the bypass air dilutes and thus lowers the temperature of the air being discharged, reducing the perception of heat coming from the dehumidifier.
Household dehumidifiers typically operate using a simple on/off control: when the humidity rises above a set point, the system turns on; when the humidity drops sufficiently below the set point, it turns off. This set point is typically adjustable by the user. Other than a full catch basin (in units without a drain line), the most common reason for a dehumidifier to stop working is because of frost or ice build-up on the evaporator.
U.S. Pat. No. 2,438,120 discloses the use of a thermostat to detect when the evaporator is approaching conditions where frost might accumulate, e.g. when the air passing through the top of the evaporator drops below 1.7° C. (35° F.), and turn off the compressor until the thermostat warms up. It shows the fan and its motor just upstream of the condenser, and has an adjustable air restriction in the form of a main shutter with rotating slats immediately after the evaporator. In addition, it discloses a bypass opening downstream of the main shutter and upstream of the fan and fan motor; the bypass opening has its own adjustable restriction in the form of a bypass shutter. Opening the bypass shutter allows some room air to enter the dehumidifier immediately after the main shutter, where it mixes with the cold dehumidified air and is pulled through the fan; the mixture of dehumidified air and bypass air then goes through the condenser and cools it.
U.S. Pat. No. 6,490,876 describes various situations where a control system detects impending or actual freezing of condensate onto the evaporator, and shuts down the compressor motor either for a predetermined interval or until freezing conditions are abated, while allowing the fan to continue running. This allows the dehumidifier to defrost, and then continue dehumidifying. Disclosed situations indicating condensate freezing include the temperature of the evaporator dropping well below freezing, dropping rapidly when just below freezing, or when the current drawn by the compressor motor drops below a particular threshold or drops rapidly from its typical operating point.